Microstructural Changes During Stress Relief Heat Treatment of Inconel 625–A106 Carbon Steel Joint

The present study investigates the effect of stress relief treatment at different temperatures (900, 1040, and 1200 °C) on the microstructure of Inconel 625 and A106 carbon steel weld joints. The results of the study show that, due to the generation of heat by welding, the bainite phase was formed in the heat-affected zone of A106 alloy. By applying stress relief treatment, even at the lowest temperature, the bainite phase was removed from the microstructure and the ferritic–pearlitic structure was formed. As the temperature of stress relief treatment increased, the grain size of the samples and the ratio of pearlite structure increased in the matrix. The morphology of the weld metal was converted from the columnar dendritic to an island state. Gradually, by increasing the temperature up to 1200 °C, austenitic grain boundary appeared in the microstructure. The SEM results confirmed the presence of complex molybdenum–niobium–rich carbides among secondary phase particles in the weld zone. In addition, an increase in the heat treatment temperature caused an increase in the concentration and dimensions of the carbides in the heat-affected zone, weld metal, and base metal of Inconel 625.